Some multi-band or other tactical radios and operate in the high frequency (HF), very high frequency (VHF) (for satellite communications), and ultra high frequency (UHF) bands. The range of some multi-band tactical radios can operate from about 2 through about 512 MHz frequency range in some non-limiting examples. The latest generation radios cover about 2.0 to about 2,000 MHz (or higher) to accommodate high data rate waveforms and less crowded frequency bands. The high frequency (HF) transmit mode is governed by standards such as MIL-STD-188-141B, while data modulation/demodulation is governed by standards such as MIL-STD-188-110B, the disclosures which are incorporated by reference in their entirety.
UHF standards, on the other hand, provide different challenges over the 225 to about 512 MHz frequency range, including short-haul line-of-sight (LOS) communication and satellite communications (SATCOM) and cable. This type of propagation can be obtained through different weather conditions, foliage and other obstacles making UHF SATCOM an indispensable communications medium for many agencies. Different directional antennas can be used to improve antenna gain and improve data rates on the transmit and receive links. This type of communication is typically governed in one example by MIL-STD-188-181B, the disclosure which is incorporated by reference in its entirety. This standard provides a family of constant and non-constant amplitude waveforms for use over satellite links.
The joint tactical radio system (JTRS) is one example of a system that implements some of these standards and has different designs that use oscillators, mixers, switchers, splitters, combiners and power amplifier devices to cover different frequency ranges. The modulation schemes used for these types of systems can occupy a fixed bandwidth channel at a fixed frequency spectrum. These systems usually utilize a memoryless modulation, such as phase shift keying (PSK), amplitude shift keying (ASK), frequency shift keying (FSK), quadrature amplitude modulation (QAM), or modulations with memory such as continuous phase modulation (CPM) and may sometimes combine them with a convolutional or other type of forward error correction (FEC) code. Minimum shift keying (MSK) and Gaussian minimum shift keying (GSMK) (together referred to as MSK or GMSK) are a form of continuous phase modulation used in the Global System for Mobile communications (GSM) and can be used with such systems. The circuits used for implementing the MSK waveform could include a continuous phase frequency shift keying (FSK) modulator.
Some of these radios use DAMA satellite communication networks, which have enjoyed widespread use in a variety of applications, such as, but not limited to military environments. In certain military applications, an established requirement issued by the Department of Defense, known as MIL-STD-188-183 and 183A, the disclosure which is hereby incorporated by reference in its entirety, sets forth interoperability standards with which (5 KHz and 25 KHz UHF) satellite communication equipment must conform. A reduced complexity example of such a SATCOM network is diagrammatically illustrated in FIG. 1 and includes a (geosynchronous) communication satellite 10 and a plurality of (mobile) terrestrial transceivers/radios 12.
DAMA is a technique that increases the amount of users that a limited “pool” of satellite transponder space can support. The ability to share bandwidth is based on the theory that not all users will require simultaneous access to communication channels. DAMA systems quickly and transparently assign communication links or circuits based on requests issued from user terminals to a network control system. When the circuit is no longer in use, the channels are immediately returned to the central pool, for reuse by others. By using DAMA, many subscribers can be served using only a fraction of the satellite resources required by dedicated, point-to-point single-channel-per-carrier networks, thus reducing the costs of satellite networking.
Existing MIL-STD-188-183 and 183A terminals require acquisition and demodulation of various Phase Shift Keying (PSK) modulation types, such as Shaped Offset Quadrature Phase Shift Keying (SOQPSK), Differential Encoded Quadrature Phase Shift Keying (DEQPSK), and Binary Phase Shift Keying (BPSK) modulation types. New MIL-STD-188-181C (Integrated Waveform) requires acquisition and demodulation of Continuous Phase Modulation (CPM) types (in addition to legacy waveforms such as BPSK, DEQPSK, and SOQPSK). The specified preamble phasing sequence for each of the modulation types is similar. The required Signal-to-Noise Ratio (SNR) requires advanced signal processing techniques to recover symbol frequency offset, phase offset, and timing.
Existing DAMA terminals and controllers acquire the modulation preamble by predefining the modem baud rate and correlating for the specific modem phasing pattern and start-of-message bit sequence. Baud is a measure of the bit rate, i.e. the number of distinct symbolic changes (signaling event) made to the transmission medium per second in a digitally modulated signal. As each symbol may stand for more than one bit of information, the amount of information sent per second is the product of the rate in baud and the number of bits of information represented by each symbol. The baud rate is equal to the symbol rate times the number of bits per symbol.
One multiband radio sold under the designation AN/PRC-117F(C) is a multiband, multimission, software-defined radio, for example, the Falcon II Manpack from Harris Corporation of Melbourne, Fla. This radio uses BPSK, DEQPSK and the SOQPSK waveforms for DAMA operation. In many of these communications systems, DAMA waveforms have a 32 Hz/sec Doppler tracking design objective to allow for airborne operation. In the radio receiver, the RF circuitry shifts or demodulates an information bearing component of a received signal back to baseband by multiplying it with a local reference of frequency FC. This carrier recovery also is termed phase tracking and must be very accurate to determine data bit values represented by received symbols. The down-conversion can be difficult because of phase variations introduced by Doppler shifting as the transmitted signal passes through a channel because of the relative motion between a transmitter and receiver in wireless systems. Also, a local reference at a transmitter could be out of phase with the local reference at the receiver and the phase error could be time varying.
At lower symbol rates, for example, at 600 and 1,200 symbol rates, the traditional phase and Doppler (frequency) offset tracking approaches suffer, especially at a lower signal-to-noise (Eb/No) ratio. It has been found that the bandwidth of a traditional tracking loop is required to be wide to track 32 Hz/sec. In some systems, however, this type of wide bandwidth tracking loop allows greater noise at lower Eb/No, which causes the loop to track off the desired offset.
To overcome these detriments, some communications devices use a phase estimator as a low pass filter while others have used a phase-locked loop (PLL) circuit with a tracking bandwidth that is set and based on a maximum offset that is allowed under the circuit and communications conditions. The phase-locked loop circuits typically incorporate a filter bandwidth reduction after the signal acquisition.
These approaches provide some remedial effect, but do not always adequately perform under all circumstances and greater enhancements to Doppler (frequency) and phase tracking are desired.